In order to reduce the polluting emission, the modern Diesel engine are conventionally equipped with a DPF, which is located in the exhaust pipe so as to retain the particulate matter (soot) contained in the exhaust gas. The operation of the DPF is characterized by its efficiency, which can be defined as the ratio of the quantity of soot retained in the DPF to the quantity of soot entering the DPF.
The current antipollution regulation does not provide any requirement about the DPF efficiency during the real road running of the motor vehicle, but only provides that the cumulative quantity of soot emitted by the motor vehicle during a test driving cycle is below a predetermined threshold. Nonetheless, it could be advisable for the On Board Diagnostic (OBD) of the motor vehicle to evaluate the DPF efficiency during a real road running, and to indicate to the driver if the DPF efficiency is below an admissible limit.
For this purpose, several studies have been already presented, which suggest evaluating the DPF efficiency by means of a soot-sensor located in the DPF outlet. As a matter of fact, these studies suggest to measure the soot concentration at the DPF outlet by means of said soot-sensor, to determine the soot concentration at the DPF inlet by means of an estimation or another soot-sensor, and to calculate the DPF efficiency as a function of these soot concentrations. However, these studies do not explain whether the above mentioned strategy can really provide a reliable DPF efficiency determination, since current in-development soot-sensors are not actually able to provide a continuous measurement of the soot concentration in the exhaust gas, but only a mean value of the soot concentration over a wide time range.
Current in-development soot sensors are conductometric sensors comprising a plurality of electrodes mutually separated on an insulating substrate provided for loading soot. As the soot is loaded on this substrate, an electrical current starts to flow between the electrodes of the soot-sensor and then gradually increases. When the current exceeds a predetermined threshold, the soot-sensor is subjected to a regeneration phase, during which the accumulated soot is burned off by means of a dedicated heater associated to the substrate, so as to get the soot-sensor ready for another soot loading phase. As a matter of fact, the operation of a soot-sensor is characterized by a continuous repetition of soot loading phases alternated by a regeneration phase.
A functional limit of this soot-sensor is that the electrical current generated during each soot loading phase becomes measurable only after a certain delay from the beginning of the soot loading, and then it rapidly increases with an exponential law. Due to this behavior, it is not possible to have a continuous measurement of the soot concentration in the exhaust gas. Conversely, it is possible to establish a reliable relationship between the soot concentration in the exhaust gas and the duration of the soot-loading phases, which is generally referred as time response of the soot-sensor.
However, it necessarily implies that the soot-sensor can only return a mean value of the soot concentration in the exhaust gas over a wide time range. As a consequence, the above mentioned strategy for determining the DPF efficiency is substantially unfeasible with the current in-development soot-sensors.
In view of the above, it is at least one object to provide a reliable strategy for evaluating the DPF efficiency during a real road running of the motor vehicle, using a soot-sensor located in the DPF outlet and overcoming the above mentioned drawbacks. At least another object is to reach this goal with a simple, rational and rather inexpensive solution. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.